Method for separating c2+-hydrocarbons from a hydrocarbon-rich fraction

ABSTRACT

Described herein is a method for separating C 2+ -hydrocarbons from a hydrocarbon-rich fraction comprising partially condensing a hydrocarbon-rich fraction, and separating the hydrocarbon-rich fraction into a gaseous fraction and a liquid fraction. These fractions are subjected to a rectification fractionation to obtain a methane-rich fraction and a C 2+ -hydrocarbon-rich fraction. The methane-rich fraction is compressed, and a partial stream of the compressed methane-rich fraction is condensed is fed as reflux to rectification fractionation. In addition, before rectification fractionation, the liquid fraction is separated into two partial streams. The first partial stream is partially evaporated and then is fed to rectification fractionation. The second partial stream is undercooled and then is fed as additional reflux to rectification fractionation.

FIELD OF THE INVENTION

The invention relates to a method for separating C₂₊-hydrocarbons from a hydrocarbon-rich fraction, in particular from natural gas, wherein:

-   -   a) the hydrocarbon-rich fraction is partially condensed,     -   b) is separated into a gaseous fraction and a liquid fraction,     -   c) these fractions are subjected to a rectification         fractionation into a methane-rich fraction and a         C₂₊-hydrocarbon-rich fraction,     -   d) the methane-rich fraction obtained in the rectification         fractionation is compressed, and     -   e) a partial stream of the compressed methane-rich fraction is         condensed and is fed as reflux to the rectification         fractionation.

A general method for separating C₂₊-hydrocarbons from a hydrocarbon-rich fraction is known from U.S. Pat. No. 5,568,737. As described therein, a partial stream of the compressed methane-rich fraction is condensed and recycled as reflux to rectification fractionation. This recycling of the partial stream is referred to as so-called light sales gas recycling. By means of this method using light sales gas recycling, the yield of C₂₊-hydrocarbons can be significantly increased, in comparison to performing the method without sales gas recycling.

The ethane product that is obtained by means of the general method is frequently used as feedstock in an olefin complex, for example a chemical plant which produces olefins by steam cracking of natural gas liquids like ethane and propane. Construction of the olefin complex is sometimes completed only after the natural gas unit, in which the generic method is implemented. Therefore, it happens that at the start-up of operation of the natural gas unit—but also for other reasons or at other times—a C₃₊-hydrocarbon-rich fraction with a high C₃ yield is obtained, rather than a C₂₊-hydrocarbon-rich fraction. The current topology of the C₂₊-separation does not allow such an approach.

Therefore, an object of this invention is to provide a general method for separating C₂₊-hydrocarbons from a hydrocarbon-rich fraction, which has an improved thermodynamic process behavior without decreasing the yield of C₂₊-hydrocarbons. Moreover, according to a further object, the method is able to be operated, at least temporarily, in such a way that a C₃₊-hydrocarbon-rich fraction can be obtained, instead of the C₂₊-hydrocarbon-rich fraction.

SUMMARY OF THE INVENTION

Upon further study of the specification and appended claims, other objects, aspects and advantages of the invention will become apparent.

To accomplish these objects, a method for separating C₂₊-hydrocarbons from a hydrocarbon-rich fraction is proposed, which is characterized in that

-   -   the liquid fraction that is obtained in process step b) is         separated into two partial streams,     -   the first partial stream is partially evaporated and is fed to         rectification fractionation, and     -   the second partial stream is subcooled and then is fed as         additional reflux to rectification fractionation.

According to the invention, a separation of the liquid fraction, obtained from the partial condensation of the hydrocarbon-rich (feedstock) fraction, into two partial streams is performed. While the first partial liquid stream is partially evaporated in a known way and then fed to rectification fractionation, the second partial liquid stream is sub-cooled and fed as additional reflux—that is in addition to the reflux formed by a partial stream of the compressed methane-rich (product or sales gas) fraction—to rectification fractionation or to the demethanizer. By contrast, in the above-mentioned state of the art method, the entire liquid fraction is fed directly to rectification fractionation. By means of the previously-described division of the liquid fraction and the use of a partial stream as an additional reflux, the amount of the reflux derived from the compressed methane-rich fraction can be reduced under the postulation of unaltered yields. As a result, this leads to a reduction in output of up to 5% at the compressor that is necessary for the compression of the methane-rich fraction. In this connection, the reduction in output is all the greater the heavier the hydrocarbon-rich feedstock fraction. The heavier the hydrocarbon rich feed stock is, the greater is the additional subcooled reflux to the demethanizer, and the lower is the energy consumption of the sales gas compressor.

Other advantageous configurations of the method according to the invention for separating C₂₊-hydrocarbons from a hydrocarbon-rich fraction are characterized in that

-   -   the first partial liquid stream is partially evaporated against         the hydrocarbon-rich fraction that is to be partially condensed,     -   the evaporation pressure of the first partial liquid stream is         variable,     -   the second partial liquid stream is sub-cooled against the         methane-rich fraction that is obtained in rectification         fractionation,     -   the sub-cooled second partial liquid stream is fed as reflux to         rectification fractionation at a point below the feed point of         the reflux formed by a partial stream of the methane-rich         fraction, and     -   at least three heating circuits at different temperature         levels—via which intermediate fractions are drawn off from the         rectification fractionation, partially evaporated and fed back         again to the rectification fractionation—are assigned to the         rectification fractionation, and at least the intermediate         fractions circulating in the two heating circuits located at the         highest temperature levels are partially evaporated against the         hydrocarbon-rich fraction that is to be partially condensed,     -   removal of the intermediate fraction circulating in the heating         circuit located at the highest temperature level is at least         temporarily interrupted,     -   during the temporary interruption of removal of the intermediate         fraction circulating in the heating circuit located at the         highest temperature level, a partial stream of the bottom         fraction, obtained from the rectification fractionation, is         partially evaporated against an external medium (reboiler) and         fed as a bottom heating to the rectification fractionation, and     -   during the temporary interruption of removal of the intermediate         fraction circulating in the heating circuit located at the         highest temperature level, the additional intermediate         fractions, in each case at the temperature level at which the         intermediate fraction circulating in the heating circuit located         at the next-higher temperature level was partially evaporated,         are partially evaporated.

BRIEF DESCRIPTION OF THE DRAWINGS

The method according to the invention for separating C₂₊-hydrocarbons from a hydrocarbon-rich fraction as well as additional advantageous configurations thereof are explained in more detail below based on the embodiment depicted in FIG. 1.

Via line 1, the hydrocarbon-rich (feedstock) fraction is fed to the heat exchanger E1 and is partially condensed in the latter against process streams that are to be heated, which will be explained in greater detail below. The hydrocarbon-rich fraction is then fed via line 1′ to a separator D and separated in the latter into a gaseous fraction 5 and a liquid fraction 2. According to the invention, the liquid fraction 2 is divided into two partial streams 2′ and 4. The first partial stream 2′ is partially evaporated in heat exchanger E1 against the hydrocarbon-rich fraction 1 that is to be condensed, and is then fed via the line sections 3 and 3′ to rectification fractionation or a demethanizer T. By means of the valves V1 and V2, the evaporation pressure can be varied, and the optimal approach can be adjusted accordingly. This is achieved by an improvement of the Q/T profile in the heat exchanger E1. By adjustment of the evaporation pressure of the first partial stream the resulting Q/T composite curve approach can be optimized thus resulting in a more energy efficient operation.

The second partial stream 4 is sub-cooled in the heat exchanger E2 against process streams, which will be explained in greater detail below, and fed via line 4′ and expansion valve V5 as additional reflux to the rectification fractionation or the demethanizer T. In this connection, this additional reflux is preferably introduced into the rectification fractionation/demethanizer at a point below the feedpoint of a reflux stream 16, which will be explained in greater detail below. The previously-described way in which this method is performed improves the Q/T profile of the heat exchanger E2 by adjusting the flow rate of the second partial liquid stream to heat exchanger E2.

The gaseous fraction 5 that accumulates in the separator D is also divided into two partial streams 6 and 7. While the first partial stream 6 is expanded in the expander X, which is coupled to the first compressor Cl that is yet to be described, and then fed via line 6′ to the demethanizer T, the second partial stream 7 is cooled in heat exchanger E2, condensed, and then fed via line 8 and expansion valve V3 to the upper region of the demethanizer T.

The C₂₊-hydrocarbon-rich (product) fraction 9 that is to be obtained is drawn off from the bottom of demethanizer T, and fed to a further use, for example, to a fractionation and then as a feedstock fraction for an olefin unit.

At the head of demethanizer T, a methane-rich fraction 10 is drawn off and heated in heat exchanger E2 against the second partial liquid stream 4 that is to be sub-cooled, as well as against additional process streams. Via line 11, this fraction is then fed to heat exchanger E1, where it is further heated against the hydrocarbon-rich fraction 1 that is to be condensed, and then fed via line 12 to the first compressor C1. In compressor C1, the pressure of the methane-rich fraction is increased by 2 to 10 bar. Then, the compressed methane-rich fraction is fed via line 13 to a second compressor C2 and is further compressed in the latter to the desired release pressure, which is preferably 40 to 60 bar. The compressed methane-rich fraction is then removed from the system via line 14 and can optionally be subjected to further compression.

A partial stream of the methane-rich fraction that is compressed to the release pressure of the compressor C2 is fed via line 15 to heat exchanger E1, cooled in the latter, partially condensed, and then fed via line 15′ to the heat exchanger E2, further condensed in the latter as well as sub-cooled, and then fed as (main) reflux, via line 16 and regulating valve V4, to the demethanizer T.

As heat exchangers E1 and E2, preferably multi-stream plate exchangers are used, but several tube bundle exchangers can also be provided. The heat exchanger or reboiler E3 that is yet to be described is preferably a tube bundle heat exchanger, which, however, is required only for the separation of C₃₊-hydrocarbons that is yet to be described.

As already mentioned above, an additional object of the method according to the invention for separating C₂₊-hydrocarbons from a hydrocarbon-rich fraction is to provide a method that can be operated, at least temporarily, in such a way that instead of the C₂₊-hydrocarbon-rich fraction 9, a C₃₊-hydrocarbon-rich fraction can be obtained.

In the embodiment, depicted in FIG. 1, of the method according to the invention, the demethanizer T has three heating circuits 20/20′, 21/21′, and 22/22′. For the sake of clarity, the (regulating) valves that are necessary in these heating circuits are not shown. Three different intermediate fractions are drawn off from the demethanizer T via the above-mentioned heating circuits, partially evaporated at different temperature levels, and then fed back again to rectification fractionation or to demethanizer T. While the intermediate fractions circulating in the two heating circuits 20/20′ and 21/21′, located at the highest temperature levels, are partially evaporated against the hydrocarbon-rich fraction 1 that is to be partially condensed in the heat exchanger E1, the intermediate fraction of the heating circuit 22/22′, located at the lowest temperature level, is partially evaporated in the heat exchanger E2 against process streams that are to be (sub-) cooled.

The temperature level of the three heating circuits lies below the temperature of the hydrocarbon-rich feedstock fraction 1 that is to be cooled or condensed in the separation of C₂₊-hydrocarbons, and thus external heating by steam or hot oil in the heat exchanger or reboiler E3 is not necessary. If a separation of C₃₊-hydrocarbons is now to be carried out, the bottom temperature of the demethanizer T increases noticeably, namely from ambient temperature to about 100° C. It is thus no longer possible to heat the demethanizer T with the hydrocarbon-rich feedstock fraction; rather, external heating would be necessary. Moreover, the entire temperature profile increases in the demethanizer T, which then performs the function of a deethanizer.

Further developing the method according to the invention, it is therefore proposed that the drawing-off of the intermediate fraction circulating in the heating circuit 20/20′ located at the highest temperature level be interrupted at least temporarily. In the meantime, the intermediate fraction of the heating circuit 21/21′, located at the medium temperature level, is partially evaporated at the temperature level at which the intermediate fraction circulating in the heating circuit 20/20′ that is located at the highest temperature level was partially evaporated. To this end, the intermediate fraction of the heating circuit 21/21′ is fed to and drawn off from the heating circuit 20/20′, shown in dashed-dotted lines, via the line sections a and b, respectively. This has the result that the intermediate fraction, which previously was partially evaporated at the cold end of the heat exchanger E1, is now partially evaporated at the warm end of the heat exchanger E1 against the hydrocarbon-rich feedstock fraction that is to be condensed.

At the same time, the intermediate fraction of the heating circuit 22/22′, located at the lowest temperature level, is partially evaporated at the temperature level at which the intermediate fraction circulating in the heating circuit 20, 20′ that is located at the medium temperature level was partially evaporated. To this end, the intermediate fraction of the heating circuit 22/22′ is fed to and drawn off from the heating circuit 21/21′, shown in dashed-dotted lines, via the line sections c and d, respectively. The intermediate fraction of the heating circuit 22/22′, which previously was partially evaporated in the heat exchanger E2, is now partially evaporated at the cold end of the heat exchanger E1 against the hydrocarbon-rich feedstock fraction that is to be condensed.

At the same time, in addition, a partial stream 9′ from the bottom of the demethanizer T—which now acts as a deethanizer—is partially evaporated in the heat exchanger or reboiler E3 against a suitable external medium, for example steam, hot oil, etc., and fed to the de(m)ethanizer T as a bottom heating. In this connection, the external medium should be able to provide heat at a temperature level of at least 100° C. The bottom heating via the reboiler E3 is permanently in operation during the operating mode: “recovery of C₃₊-hydrocarbons.” During the operating mode: “recovery of C₂₊-hydrocarbons,” the reboiler E3 can be used as a so-called trim-reboiler.

The previously-described way in which this method is performed thus makes it possible to use the method according to the invention for separating C₂₊-hydrocarbons from a hydrocarbon-rich fraction temporarily as a method for separating C₃₊-hydrocarbons, whereby the yield of C₃-hydrocarbons remains consistently high.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding German patent application DE 10 2012 017 486.1, filed Sep. 4, 2012, are incorporated by reference herein. 

1. A method for separating C₂₊-hydrocarbons from a hydrocarbon-rich fraction, said method comprising: a) partially condensing the hydrocarbon-rich fraction, b) separating the partially condensed hydrocarbon-rich fraction into a gaseous fraction and a liquid fraction, c) subjecting said gaseous fraction and said liquid fraction to a rectification fractionation to produce a methane-rich fraction and a C₂₊-hydrocarbon-rich fraction, d) compressing the methane-rich fraction obtained from the rectification fractionation is compressed, and e) condensing a partial stream of the compressed methane-rich fraction and feeding the condensed partial stream of the compressed methane-rich fraction as reflux to the rectification fractionation, wherein said liquid fraction (2) obtained in b) is separated into a first partial stream (2′) and a second partial stream (4), said first partial stream (2′) is partially evaporated (E1) and then fed to the rectification fractionation (T), and said second partial stream (4) is subcooled (E2) and then fed as additional reflux (4′) to the rectification fractionation (T).
 2. The method according to claim 1, wherein said hydrocarbon-rich fraction is from natural gas.
 3. The method according to claim 1, wherein said first partial stream (2′) is partially evaporated (E1) against the hydrocarbon-rich fraction (1) that is to be partially condensed.
 4. The method according to claim 1, wherein the evaporation pressure of the first partial stream (2′, 3) is variable (V1, V2).
 5. The method according to claim 1, wherein said second partial stream (4) is sub-cooled (E2) against the methane-rich fraction (10) obtained from the rectification fractionation (T).
 6. The method according to claim 5, wherein the sub-cooled second partial stream (4′) is fed as reflux to the rectification fractionation (T) at a feed point below the feedpoint of the condensed partial stream of the compressed methane-rich fraction used as reflux (16).
 7. The method according to claim 1, wherein three heating circuits—via which intermediate fractions are drawn off from the rectification fractionation, partially evaporated and fed again to the rectification fractionation—are assigned to the rectification fractionation, and the intermediate fractions circulating in the two heating circuits located at the highest temperature levels are partially evaporated against the hydrocarbon-rich fraction that is to be partially condensed.
 8. The method according to claim 1, wherein three heating circuits—via which intermediate fractions are drawn off from the rectification fractionation, partially evaporated and fed again to the rectification fractionation—are assigned to the rectification fractionation, and the intermediate fractions circulating in the two heating circuits located at the highest temperature levels are partially evaporated against the hydrocarbon-rich fraction that is to be partially condensed, removal of the intermediate fraction circulating in the heating circuit (20, 20′) located at the highest temperature level is at least temporarily interrupted, during the temporary interruption of removal of the intermediate fraction circulating in the heating circuit located at the highest temperature level, the intermediate fraction of the heating circuit (21, 21′) located at the medium temperature level is partially evaporated (E1) at the temperature level at which the intermediate fraction circulating in the heating circuit (20, 20′) located at the highest temperature level was partially evaporated, during the temporary interruption of removal of the intermediate fraction circulating in the heating circuit located at the highest temperature level, the intermediate fraction of the heating circuit (22, 22′) located at the lowest temperature level is partially evaporated (E1) at the temperature level at which the intermediate fraction circulating in the heating circuit (20, 20′) located at the medium temperature level was partially evaporated, and during the temporary interruption of removal of the intermediate fraction circulating in the heating circuit located at the highest temperature level, a partial stream (9′) of a bottom fraction (9) obtained from the rectification fractionation (T) is partially evaporated (E3) against an external medium and as a bottom heating to the rectification fractionation (T).
 9. The method according to claim 1, whereby At least three heating circuits—via which intermediate fractions are drawn off from the rectification fractionation, partially evaporated and fed again to the rectification fractionation—are assigned to the rectification fractionation, and At least the intermediate fractions circulating in the two heating circuits located at the highest temperature levels are partially evaporated against the hydrocarbon-rich fraction that is to be partially condensed, removal of the intermediate fraction circulating in the heating circuit (20, 20′) located at the highest temperature level is at least temporarily interrupted, during the temporary interruption of removal of the intermediate fraction circulating in the heating circuit located at the highest temperature level, a partial stream of a bottom fraction obtained from the rectification fractionation is partially evaporated against an external medium and is fed as a bottom heating to the rectification fractionation, and during the temporary interruption of removal of the intermediate fraction circulating in the heating circuit located at the highest temperature level, the other two intermediate fractions, in each case at the temperature level at which the intermediate fraction circulating in the heating circuit located at the next-higher temperature level was partially evaporated, are partially evaporated. 